Carbon has unique properties that are essential for life on Earth.
That means Argon atoms which are normally just gaseous become trapped in solid igneous rock.So we know that 100% of all the Argon atoms in an igneous rock got there from decaying Potassium, after it solidified.Once the lava solidifies the molecular/crystaline structure becomes locked.But some Potassium isotopes will decay over time to Argon anyway.Furthermore, different types of plants discriminate differently.
This also has to be corrected for. Second, the ratio of C in the atmosphere at that time to be estimated, and so partial calibration of the “clock” is possible.
None of this is based on speculative assumptions -- this is extremely hard core, and verified science, that is basic chemistry and basic nuclear physics.
When scientists say that a fossil has been dated to 65 million years or whatever, they are not guessing.
A stronger magnetic field deflects more cosmic rays away from the Earth.
Overall, the energy of the Earth's magnetic field has been decreasing, so more C is being produced now than in the past.
Accordingly, carbon dating carefully applied to items from historical times can be useful.